A new study by researchers at MIT shows that there is enough capacity in deep saline aquifers in the USA to store at least a century’s worth of carbon dioxide emissions from the nation’s coal-fired powerplants. Though questions remain about the economics of systems to capture and store such gases, this study addresses a major issue that has overshadowed such proposals.

Coal-burning powerplants account for about 40 percent of global carbon emissions, so climate change “will not be addressed unless we address CO2 emissions from coal plants,” says Ruben Juanes, the ARCO Associate Professor in Energy Studies in the Department of Civil and Environmental Engineering. “We should do many different things” such as developing new and cleaner alternatives, he says, “but one thing that’s not going away is coal,” because it’s such a cheap and widely available source of power.

Efforts to curb greenhouse gases have largely focused on the search for practical, economical sources of clean energy, such as wind or solar power. But human CO2 emissions are now so vast that many analysts think it’s unlikely that these technologies alone can solve the problem.

Some have proposed methods for capturing fossil fuel emissions, then compressing and storing them in deep geological formations. This approach is known as carbon capture and storage, or CCS.

One of the most promising places to store the gas is in deep saline aquifers: those more than half a mile underground, far below the freshwater sources used for human consumption and agriculture. But estimates of the capacity of such formations in the USA have ranged from enough to store just a few years’ worth of emissions up to many thousands of years’ worth.

The reason for this huge disparity in estimates is two-fold. Firstly, because deep saline aquifers have no commercial value, there has been little exploration to determine their extent. Secondly, the fluid dynamics of how concentrated, liquefied carbon dioxide would spread through such formations is very complex and hard to model. Most analyses have simply estimated the overall volume of the formations, without considering the dynamics of how the CO2 would infiltrate them.

The MIT team modelled how the carbon dioxide would percolate through the rock – accounting not only for the ultimate capacity of the formations, but the rate of injection that could be sustained over time.

“The key is capturing the essential physics of the problem,” says graduate student Michael Szulczewski, “but simplifying it enough so it could be applied to the entire country.” That meant looking at trapping mechanisms in the porous rock at a scale of microns, then applying that knowledge to formations spanning hundreds of miles.

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When liquefied CO2 is dissolved in salty water, the resulting fluid is denser than either of the constituents, so it naturally sinks. It’s a slow process, but “once the carbon dioxide is dissolved, you’ve won the game,” Juanes says, because the dense, heavy mixture would never escape back into the atmosphere.

While this study did not address the cost of CCS systems, many analysts have concluded that they could add 15 to 30 percent to the cost of coal-generated electricity, and would not be viable unless a carbon tax or a limit on carbon emissions was put in place.

While uncertainties remain, “I really think CCS has a role to play,” Juanes says. “It’s not an ultimate salvation – it’s a bridge – but it may be essential because it can really address the emissions from coal and natural gas.”

While this analysis focused on the USA, similar storage capacities are likely to exist around the world.